Circuits for generating a reference voltage, also known simply by the term “voltage reference circuits”, are circuits that play a vital role in various types of integrated circuits. In particular, a voltage reference circuit is capable of generating at least one electrical quantity with high accuracy and great stability, which quantity may be used as reference in various types of circuit blocks such as analog-to-digital converters, voltage regulators, measuring circuits, and so on. A voltage reference circuit is, therefore, provided with specific features such as good thermal stability and good electrical noise rejection, so as to be capable of providing an output voltage whose value is as independent as possible from voltage supply variations and from temperature changes of the circuit wherein it is integrated.
A class of voltage reference circuits widely known, with the features mentioned above, is the so-called bandgap voltage reference circuits class, or simply bandgap circuits. Briefly, a bandgap circuit exploits the forbidden band potential of silicon to generate an accurate reference voltage that is independent of the circuit operating temperature. The operation principle of a bandgap circuit is based on obtaining a bandgap voltage VBG that is (almost) independent of the circuit operating temperature by means of a bipolar transistor that implements the relation VBG=VBE+nVT, where VBE is the voltage between the base terminal and the emitter terminal of the bipolar transistor, VT is the thermal voltage (equal to kT/q, where k is the Boltzmann constant, T is the absolute temperature, and q the electron charge), and n is a multiplicative parameter calculated to obtain the desired compensation of the temperature variations of the voltage VBE. The voltage VBE between the base and emitter of a bipolar transistor decreases as the temperature increases—in the jargon, the voltage VBE is a quantity of the CTAT (Complementary To Absolute Temperature) type—, while the thermal voltage appears to be proportional to the temperature itself—in the jargon, the thermal voltage VT is a quantity of the PTAT (Proportional To Absolute Temperature) type.
According to an approach known in the state of the art, the bandgap voltage VBG may be generated by forcing a current Iptat provided by a current generator in a first reference circuit element comprising a transdiode coupled bipolar transistor, and mirroring the current Iptat in a second reference circuit element formed by a series of a resistor and a second transdiode coupled bipolar transistor having an emitter area different from that of the first bipolar transistor. Coupling the first reference circuit element and the second reference circuit element with respective input terminals of a high gain operational amplifier, and using the output of such operational amplifier to control the generator of the current Iptat, a negative feedback loop is established, which bears the first and second reference circuit elements voltages to a same value. With such a configuration, the current Iptat is found to be:Iptat=[ln(L1/L2)*(KT/q)]/Re, where L1 and L2 are parameters proportional to the emitter areas, respectively, of the first bipolar transistor and of the second bipolar transistor, while Re is the resistance of the resistor comprised in the second reference circuit element; as may be seen from the equation, this current appears to be of the PTAT type, being proportional to the absolute temperature T. The current Iptat is then forced into a third reference circuit element comprising an element characterized by an electrical quantity of the CTAT type for generating the bandgap voltage VGB.
A major drawback that afflicts a configuration of this type is the extreme variability of the common-mode voltage of the operational amplifier input terminals. Indeed, this voltage being dependent from the base-emitter voltages VBE of the bipolar transistors included in the first and second reference circuit elements, it may vary, for example, in a range between 0.3 and 0.8 Volts depending on temperature and tolerances of the manufacturing process. Consequently, the operational amplifier is designed to handle the large input signal excursions without compromising the proper voltage reference circuit operation. However, this may be very difficult if the supply voltage has a reduced value, as happens in the circuits integrated using advanced CMOS (Complementary Metal Oxide Semiconductor) technologies. For example, in the 90 nm CMOS technology, the power supply has a nominal value equal to 1.2 Volts; this value may actually decrease until reaching 0.9 Volts when the circuit has been designed to operate during stand-by phases in order to minimize losses due to the leakage currents presence. In these cases, the common-mode voltage excursions due to temperature change may be too large, and the transistors of the operational amplifier input stage may be forced to operate in the triode operation region, and thus the amplifier may not operate correctly.
In order to solve the above mentioned drawbacks, a solution known in the art provides for using an operational amplifier whose input stage includes n-channel MOS transistors with reduced threshold voltage. However, although this allows the operational amplifier to operate correctly even in the presence of high excursions of the common-mode voltage, forming MOS transistors with reduced threshold voltage may require an additional lithography mask, and this may imply an increase in the whole circuit production costs.
According to a further solution known in the art, the common-mode voltage value is increased by introducing resistors in series with the first and second reference circuit elements and using the voltage drops that are generated as a result of the current Iptat flowing in these resistors. Nevertheless, the problem of the common-mode voltage excursion as a function of temperature may not be resolved; if the amplifier would be supplied with a low supply voltage value, with this solution the common-mode voltage may in fact exceed the supply voltage itself, compromising the proper functioning of the amplifier.